Methyl acetate is used industrially in petrochemical processes, particularly as a feedstock for the production of commodity chemicals such as acetic acid and acetic anhydride.
Crystalline aluminosilicate zeolites have been found to catalyse the carbonylation of dimethyl ether to produce methyl acetate. For example, WO 2006/121778 describes a process for the production of a lower aliphatic ester of a lower aliphatic carboxylic acid by carbonylating lower alkyl ethers with carbon monoxide in the presence of a mordenite or ferrierite catalyst under substantially anhydrous conditions.
In U.S. Pat. No. 7,465,822 it is demonstrated that zeolites for the carbonylation of dimethyl ether to produce methyl acetate contain at least one 8-member ring channel such as those of framework type MOR, FER, OFF and GME. By contrast, zeolites not containing 8-member ring channels, such as ZSM-5 (framework type MFI), were shown to provide poor catalytic performance for this reaction.
An important aspect of any catalytic process is the performance of a catalyst when exposed to normal process conditions. The improvement of catalytic performance in carbonylation reactions is a continuous objective of process and catalyst development.
A disadvantage associated with the use of zeolites as catalysts for carbonylation processes is that they deactivate over time with a commensurate decrease in the production rate of carbonylation products. Without being bound by theory, it is believed that the deactivation of zeolite catalysts in processes for the carbonylation of dimethyl ether may be attributable to side reactions of dimethyl ether leading to the formation of hydrocarbonaceous (coke) deposits on the catalyst surface. These deposits restrict access to catalytic active sites and eventually the production rate of carbonylation products is sufficiently reduced as to necessitate replacement or regeneration of the catalyst. Processes for regenerating zeolite catalysts are disclosed for example in WO 2009/077745 and WO 2009/077739.
WO 2009/077745 describes a process for the in-situ regeneration of a mordenite catalyst in the carbonylation of a carbonylatable reactant such as dimethyl ether to form methyl acetate, in which the catalyst is regenerated by contacting the catalyst with a regenerating gas comprising a molecular oxygen-containing gas and an inert diluent at a total pressure in the range 1 to 100 bar and at an molecular oxygen-containing gas partial pressure such that the temperature of the catalyst is maintained within the range 225° C. to 325° C.
WO 2009/077739 describes a process for the in-situ regeneration of a zeolite catalyst for the production of methyl acetate by contacting a carbonylatable reactant such as dimethyl ether with carbon monoxide in the presence of the catalyst, ceasing contact of the catalyst with the carbonylatable reactant, regenerating the catalyst with a regenerating gas selected from hydrogen and a mixture of hydrogen and carbon monoxide at a temperature in the range 250 to 600° C., terminating the hydrogen regenerating step and resuming contact of the catalyst with the carbonylatable reactant and carbon monoxide.
A further disadvantage of zeolite catalysed carbonylation reactions of dimethyl ether to form methyl acetate is that the presence of water inhibits such reactions and thus typically the reactants and catalyst are dried prior to use in the carbonylation reaction.